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Many severely hypoxic cells fail to initiate DNA replication, but the mechanism underlying this observation is unknown. Specifically, although the ataxia-telangiectasia-rad3 related (ATR) kinase has been shown to be activated in hypoxic cells, several studies have not been able to document down-stream consequences of ATR activation in these cells. By clearly defining the DNA replication initiation checkpoint in hypoxic cells, we now demonstrate that ATR is responsible for activating this checkpoint. We show that the hypoxic activation of ATR leads to the phosphorylation-dependent degradation of the cdc25a phosphatase. Downregulation of cdc25a protein by ATR in hypoxic cells decreases CDK2 phosphorylation and activity, which results in the degradation of cdc6 by APC/CCdh1. These events do not occur in hypoxic cells when ATR is depleted, and the initiation of DNA replication is maintained. We therefore present a novel mechanism of cdc6 regulation in which ATR can have a central role in inhibiting the initiation of DNA replication by the regulation of cdc6 by APC/CCdh1. This model provides insight into the biology and therapy of hypoxic tumors. [ABSTRACT FROM AUTHOR]

Studies in model organisms have contributed to elucidate multiple levels at which regulation of eukaryotic DNA replication occurs. Cdc7, an evolutionarily conserved serine–threonine kinase, plays a pivotal role in linking cell cycle regulation to genome duplication, being essential for the firing of DNA replication origins. Binding of the cell cycle‐regulated subunit Dbf4 to Cdc7 is necessary for in vitro kinase activity. This binding is also thought to be the key regulatory event that controls Cdc7 activity in cells. Here, we describe a novel human protein, Drf1, related to both human and yeast Dbf4. Drf1 is a nuclear cell cycle‐regulated protein, it binds to Cdc7 and activates the kinase. Therefore, human Cdc7, like cyclin‐dependent kinases, can be activated by alternative regulatory subunits. Since the Drf1 gene is either absent or not yet identified in the genome of model organisms such as yeast and Drosophila, these findings introduce a new level of complexity in the regulation of DNA replication of the human genome. [ABSTRACT FROM AUTHOR]

Studies in model organisms have contributed to elucidate multiple levels at which regulation of eukaryotic DNA replication occurs. Cdc7, an evolutionarily conserved serine-threonine kinase, plays a pivotal role in linking cell cycle regulation to genome duplication, being essential for the firing of DNA replication origins. Binding of the cell cycle-regulated subunit Dbf4 to Cdc7 is necessary for in vitro kinase activity. This binding is also thought to be the key regulatory event that controls Cdc7 activity in cells. Here, we describe a novel human protein, Drf1, related to both human and yeast Dbf4. Drf1 is a nuclear cell cycle- regulated protein, it binds to Cdc7 and activates the kinase. Therefore, human Cdc7, like cyclin-dependent kinases, can be activated by alternative regulatory subunits. Since the Drf1 gene is either absent or not yet identified in the genome of model organisms such as yeast and Drosophila, these findings introduce a new level of complexity in the regulation of DNA replication of the human genome. [ABSTRACT FROM AUTHOR]